Manufacture of double-base propellant grains



2,946,673 MANUFACTURE OF DOUBLE-BASE PROPELLANT GRAINS Vernon R.Grassie, Kennett Square, Pa., assignor to Hereules Powder Company,Wilmington, Del., a corporaiion of Delaware No Drawing. Filed Dec. 29,1955, Ser. No. 556,058

7 Claims. (11. s2-.s

This invention relates to cast double-base propellant grains and to themanufacture thereof.

Relatively large grains 'of solid propellant suitable for propellingmilitary rockets or for actuating sizeable jet devices such asjet-assisted take-oft units for airplanes are currently produced by oneof several casting techniques, since solventless extrusion, althoughquite feasible for grains up to about 7 or 8 inches in diameter,requires excessively massive and expensive equipment to produce largerdiameter grains. Briefly, the casting technique involves bringingtogether in a mold, with or without a beaker, depending on the typegrain desired, predetermined amounts of a suitable single-basepropellant casting powder in dense granular form and of casting liquidcomprising nitroglycerin and suitable desensitizing plasticizers, andthen curing the resultant mixture to produce a cast grain of double-basepropellant. During the curing operation the casting liquid swells andcolloidizes the granules of single-base propellant casting powder andconsolidates and welds the whole mass in the mold intosingle-base'propellant, a s heretofore required. Other must then besubjected-'to removal of solvent before they'are suitable'for use ascastingpowders.

Therefore, the primary object of this invention is to provide animproved and simplified method ofproducing cast double-base propellantgrains, which eliminates the necessityof initially converting fibrousnitrocellulose into fully colloided uniformly shaped andsi zed grains ofobjects of the invention will appear hereinafter, the novel features andcombinations being set forth in the appended claimsg f 7 ,Generally, thepresent invention comprises agitating fibrous nitrocellulose 'inanaqueous bath containing organic liquid solvent havingactive-solvent-power for said nitrocellulose,- the solvent being presentin amount to soften and destroy the fibrous structure of the nitro- 3cellulose without dissolution thereof, continuing agitation a solidunitary homogeneous mass conforming-to the shape of the mold. Thecasting is cured in the mold for a suitable length of time, usually atan elevated temperature on the order of 140 F., and after curing isready for use in the rocket assembly for which it is designed.

When the grain design requires restricted outer surwith removal ofsubstantially all the solvent until smooth,

hardened, densified and irregular particles of nitrocellulose areobtained, removing substantially all water from the. resulting densifiednitrocellulose particles, bringing together in a mold predeterminedamounts of the resultingdry densified nitrocellulose particles and ofcasting liquid to produce a solid unitary cast double-basepropellant'grain, and curing the resulting'propellant grain.

More specifica1ly, in carrying out this invention, waterwetnitrocellulose, after stabilization treatment, is mixed and agitatedwith alarge quantity-of water and preferably asmallquantity of anorganic liquid having active solvent power for nitrocellulose and havingan' appreciable vapor pressure at or below the boiling point, of

water; The organic liquid solventis preferentially absorbedby thenitrocellulose whereby the nitrocellulose is softened and the fibrousstructure thereof is destroyed. However, the nitrocellulose is notdissolved and the particles thus modifiedare hardenedand densified byboiling oil the solventor by dilution of themixture with water.Agitation is maintained until hardening is, complete to prevent:agglomeration. Substantially all r the remaining solvent isremoved fromthe nitrocellulose bytdistillation.

. The resulting smooth, hardened, densifiediand irregular faces, as forexample, in case the grain is to burn only' from the ends or from axialperforations, it is customary to cast in a beaker. This beaker consistsof a cellulose acetate plastic tube that is prefabricated by extrusion,or wrapping on a mandrel. V t the mold before adding casting powder.During the curing operation the beaker and propellant fuse together, andthe beaker becomes an integral part of the cured propellant grain. 7 V

The principal disadvantage of castingfmethods employed heretoforeresides in the tedious and expensive process of producing suitableinitial single-base propellant casting powders in dense granular form.It is well known, of course, that cast propellant grains cannot bemanufactured directly from fibrous nitrocellulose because a great enoughmass of fibrous nitrocellulose cannot be forced into the casting mold,and because the solvent action of the casting liquid on fibrousnitrocellulose is so rapid that dissolving and plugging occur beforeuniform distribution of casting liquid through the The beaker. isassembled to particles of nitrocellulose are then dried, and apredetermined quantity 'of the dry dense particles, preferably A predeterminedquantity of casting liquid comprising nitroglycerin and suitabledesensitizing plasticizers, with or without stabilizers} and/or otheradjuvants included to control orregulate burning rateof the resultingpropellant'grain when 'fired, is then introduced into the mold tocompletely fill all voids between the particles of nitro-..

cellulose and to just cover the nitrocellulose particles. The mixture ofdensified nitrocellulose particles and casting ,liquidis, then cured inthe mold, preferably at an elevated temperature, for a suitablelength oftime, such whole mass can be achieved. Accordingly, it has beennecessary heretofore to convert fibrous. nitrocellulose into small,dense, uniform, fully colloidized grains of single-base propellant ofapproximately 25 'mesh' size.

This involves alcohol dehydration of the water-wet fibrous Initrocellulose, mastication of the dehydrated nitrocellulose withvolatile solvents to form av completely colloided mass, extrusion of theuniformly colloided ma'ss'to form uniformly thin strands which are thencut'into uniform lengths. The fully colloided particles thus obtained jas, forexample, 96 hours at F. During the curing operation the particlesof nitrocellulose absorb all of the casting liquid and, due to the,solvent action of the cast- I ing liquid .on the nitrocellulose, becomeswollen and fully colloidized, and thewhole mass in themold consolidates and welds together into a solid,-unitary, homogeneouscolloided structure conforming to the shape of the mold. Aftercuring,the cast propellant grain is ready for use in the rocket assembly forwhich it is designed.

The'gene'ralnature of the invention having been set forth, the followingexamples are presented a's specific illustrations thereof. It will beunderstood, of course,

that the inventionis not limited to the examples butis susceptible todifferent modified embodiments which 7 come within the scope of theclaims.

Patented July 26, 1960 V I 3 EXAMPLE 1 Five thousand grams (5,000), drybasis, of water-wet fibrous nitrocellulose of 12.6% nitrogen content,after conventional preliminary stabilization treatment, but beforejordaning, was 'slurried with 45,000 grams of water in a 20-gallonjacketed vessel fitted with a condenser and a 12-inch diameter turbineagitator driven at 200 rpm. The mixture was heated to 91 C. by employingsteam in the jacket of the vessel. Nine thousand five hundred ten grams(9,510) of methyl isobutyl ketone was then added to the heated aqueousnitrocellulose slurry under reflux conditions with agitation, requiringabout 15 minutes. Distillation was then carried on with continuedagitation for 90 minutes, during which time the distillation temperaturegradually increased to the boiling point of water. Distillation withagitation was continued for an additional 140 minutes. Substantially allof the solvent was removed by distillation, and the nitrocellulose wastransformed, without being dissolved, from its initial fibrous stateinto smooth, hardened, densified, irregular particles having a bulkdensity (dry basis) of 41 pounds per cubic foot. Nitrocellulose in itsinitial fibrous state, derived from either cotton linters or wood pulp,has a bulk density of about 8 to about 15 pounds per cubic foot.

The densified product thus obtained was screened to remove particleslarger than 20 mesh, and that portion of the product which passedthrough a 20 mesh screen was tray dried in warm air at about 140 F. toremove substantially all water. The dried product containedsubstantially less than 1% total volatile by weight. Approximately 150grams of the resulting dry densified nitrocellulose particles was thenloaded into a tubular cellulose acetate beaker 12 inches long and 1 /2inches inside diameter, suitably supported in a readily removable moldof rigid material equipped with a valved Vacuum line at the bottom and avalved casting liquid line at the top. The loaded mold was placed underpressure of less than mm. of mercury absolute for 16 hours. Whilemaintaining vacuum on the loaded mold, casting liquid under a pressurehead of 1 to 3 pounds per square inch gauge was introduced into theloaded evacuated mold until all interstices between particles ofnitrocellulose were filled with casting liquid and some casting liquidhad overflowed into the vacuum trap, whereupon the supply of castingliquid was cut ofi. Vacuum was-kept on -the mold during theentirecasting process, and when the casting was complete the castingliquid and vacuum lines were disconnected from the mold. The casting wasthen cured in the mold at 140 F. for 96 hours.

The finished casting upon removal from the-mold was hard and wellconsolidated, and contained approximately 150 grams of casting liquid,so that the ratio of nitrocellulose to casting liquid was essentially lto 1 ona weight basis. The composition of the casting liquid used was asfollows:

Parts by weight Nitroglycerin 72 Triacetin (glyceryl triacetate) 27Z-nitrodiphenylamine l The cured propellant grain after inspection wascut into 1% inch lengths which were fired successfully in a conventionalvented vessel having a nozzle diameter of P; designates pressure takenone 'se'e'ond after ignition. P, designates pressure at midpoint ofburning. P; designates pressure taken one second beibre end of burning.

The vented vessel employed in these tests and in other tests hereinafterset forth was a conventional stationary rocket motor equipped withautomatic devices for making a continuous record in the form of a graphof pressures developed during burning as a function of time. Such ventedvessels are standard equipment in all ballistic laboratories engaged inevaluation of rocket or gas generator solid propellant grains.

Attention is directed at this point to the fact that the nitrocelluloseemployed in this example was taken after preliminary stabilizationtreatment, but before jordaning. Accordingly, this means that thenitrocellulose was not subjected to the usual jordaning, followed'by thecustomary four-hour soda boil, a water wash, a two hour neutral boil, awater wash, a one-hour neutral boil, 2. water wash, a one-hour neutralboil, and final washing with water. The full customary stabilizationtreatment of nitrocellulose intended for use in solid propellant grainsor smokeless powder may be summarized briefly as follows:

The nitrocellulose, after centrifuging and drowning in water, issubjected to a 24-hour acid boil in an aqueous bath containingapproximately 0.35% acidity calculated as H 50 This is followed by awater wash, an eighthour neutral boil in water, a water wash, anothereighthour neutral boil, a water wash, followed by jordaning, a four-hoursoda boil, a water wash, a two-hour neutral boil, a water wash, aone-hour neutral boil, a water wash, a one-hour neutral boil, and finalwashing with water. This conforms with IAN-N-244, July 31, 1945, JointArmy-Navy Specification, Nitrocellulose, page 3. The fact that jordaningand final stabilization after jordaning can be dispensed with inaccordance with this invention is an important advantage of thisinvention, as will be evident from Example 2 following.

EXAMPLE 2 A sample of fibrous nitrocellulose of 12.6% nitrogen contentafter nitration, centrifuging, and drowning in water was given a 12-houracid boil (0.3 to 0.4% acidity as H water washed, boiled in water foreight hours, and water washed. The 134.5 C. heat stability test was 10minutes. Viscosity was 25.2 seconds. Fully stabilized nitrocelluloseintended for use in smokeless powder or propellant grain manufacturenormally has a stability of 30 minutes in the 134.5 C. heat stabilitytest. Five thousand grams (5,000) of the above partially stabilizednitrocellulose Was slurried with 45,000 grams of water in a 20-gallonjacketed vessel fitted with a condenser and a 12-inch diameter turbineagitator driven at 200 r.p.m. The slurry was heated to 91 C. byemploying steam in the jacket of the vessel. Nine thousand five hundredgrams (9,500) of methyl is'obutyl ketone containing 50 grams ofZ-nitrodiphenylamine was then added to the heated aqueous nitrocelluloseslurry under reflux conditions with agitation, requiring about fourminutes. Distillation was then carried on with continued agitation for45 minutes, during which time the distillation temperature graduallyincreased to C. Distillation was continued for an additional tenminutes. Substantially all of the solvent was removed by distillationand the nitrocellulose was transformed, without being dissolved, fromits initial fibrous state into smooth, hardened, densified irregularparticles having a 134.5 C. heat stability test of 73 minutes and aviscosity of 11 seconds. Ordinary single-base propellant, prepared fromnitrocellulose fully stabilized by customary procedure has a 134.5" C.heat stability test of 45 to 50 minutes.

EXAMPLE 3 Five thousand grams (5,000), dry basis, of water-wet fibrousnitrocellulose of 12.6% nitrogen content, after jordaning and finalstabilization as hereinbefore set forth in the final paragraph inExample 1, was slur-ried with 45,000 grams of water in a 20-gallonjacketed vessel fitted with a condenseri and a 12-inch diameter turbineagitator driven at 200 r.p.m. The mixture was heated to 91 C. byemploying steam in the jacket of the vessel. Ten thousand two hundred'forty grams (10,240) of methyl isobutyl ketone containingSO grams of2-nitro- EXAMPLE .4 a Five thousand grams (5,000), .dry. basis, .ofwaterwet fibrous. nitrocelluloseof' 12.6% nitrogen content, afterjordaning and final stabilization as hereinbefore set forthd1phenyl'amme was then added to the heated aqueous m the finalparagraph.m Examp 1e Slumgd wlth nitrocellulose Slurry under refluxonditions with a 45,000 grams. of water ma 20-gallon acketed vesselfitted flon re r g1 with-a condenser and a 12-inch turbine agitatordriven quirmg about 15 minutes. DlStlHatlOn'WaS then I v 3 carried 0 at200 r.p.m. The mixture was heated to 91 C. by n with continued agitationfor 90 minutes, durin employing steam 1n the acket ofthe vessel. Eightthoug which time the dlstillatlon temperature gradually m- Y creased to100 C Distillation with agitation was con- Sand elght hundreq cieventygrams (837.0) i methyl tinued for an additional 30 minutes Substantiallyall of butyl ketone contalmng 50 grams of z'mtrodnihenylamine thesolvent was removed by distillation and the nitrowas then'added to theheated aqueous nitrocellulose cellulose was transformed without beingdissolved from slurry nude? reflux cqnliifiqns with agitation" requu. gr r about six minutes. Distlllationwas then carried on with 1ts lmtlalfibrous state into smooth, hardened, densified, 15 7 continued agitationfor 85 minutes, during which tnne the irregular particles. The water-Wetdenslfied product was tin t t d all d 100 C screened with the followingresults: v a 9 empera i gTa u y mclieiase Distillation was continued foran additional 30 minutes.

Substantially all of the solvent was removed by distil- Scteen Mesh sizeg gg lation, and the nitrocellulose was transformed, without ClassifiedSample No. Weight Pounds being, dissolved, from its imtial fibrous statemto smooth, c fg hardened, densified irregular-particles. :Ihe;water-wetThrough On Foot product was divided into three portions. The firstportion was screened through a 10 mesh screen, and all material 10 14 2939 passing through the screen was designated as Sample 1. $3 i; ii Thesecond portion was screened through a 14 mesh 35 65 3 1 48 screen, andall material passing through the screenwas 7 designated as Sample 2. Thethird portion was screened The various screened fractions were then eachtray through a 20 mesh screen, and all material passing dried in warmair at about 140 F. to a total volatile through the screen wasdesignated as Sample 3. content substantially less than 1% by weight,and castings These three samples were then dried and cast into were madefollowing the procedureset forth in Example propellant grainssubstantially as set forth in Example 1. l, employing the diiferentclassified samples individually, Data relative to particle sizedistribution within each and also blends of these. Pertinent data on thecastings sample, bulk density, and vented vessel firing data appearmade, including vented vessel firing data, appear in Table in Table 2following; 1 following: 1 V Table 1 Vented Vessel firing Data LengthoiBulk Castin Density, Grain Out DensitiedNitrocelluloselurticles LbJOu.PressureinLb./Sq. In. BurningRate Noteson Castings Employed Ft. aBurning lWeb Vented Time (Inches), essel Fir- P Pz' P; Max. (Seconds)Burning Time ing (Inches) I (Seconds) 10-14 Mesh 39 Not tested forfiring characteristics Solidgn te I DOIOS 14-20 Mesh... 41 348 301 235610 4.10 0.122 Solid and'well 1' consolidated. 20-35 Mesh 44 344 304 336632 8.14 '0.123 do 2 -65 Mesh 48 Samples too small 0 make casting Equalparts by wt. of 10-14 224 151 151 224 11.33 0.088 Solid and well 2 Mesh;14-20 Mesh. consolidated. Equal parts by wt. of 14-20 43 233 178 202 23310.93 0.091, .-.--do 2 Mesh; 20-35 Mesh. 7 Equal parts by wt. of 10-1444 286 211 233 286 9.25 0.108 -..-do 2 Mesh; 20-35 Mesh.

Composition of casting liquid in all cases was identical to that setforth in Example 1, and in all cases the ratio of densifled nitrocellulose particles to casting liquid in the cast propellant grains wasapproximately 1 to 1 by weight. The orifice employed in the ventedvessel firing tests was 0.100 inch diameter in all tests set forthabove.

Table 2 Particle Size Distribution Vented Vessel Firing Data lgngttih oias 11 Sample 1 a Grain Out Desig- Through Through Through Bulk Pressurein I la/Sq. In. Burnigg Rate Notes on Employed nation Me 14 Mesh 20 MeshThrou h Density, I Burning 96 sh Castings Invented on 14 on 20 on 35 '35Mes Lb./Cu. Time (Inches) Vessel. Mesh, Mesh, Mesh, Percent Ft.(Seconds) Burning T me Firing. Percent Percent. Percent 4 P1 7 P: .PMax. 1 (Sec) (Inches) 25 3s 34 3 44 325 436 331 570 6. s2 0.147 solidand well: Q 2 i consolidated. 1 7 None 51 45 4 46 341 451 3562 626 6.96t 0.144 do; 2 None None 92 8 46 352 400 188 512 7.71 0.130 do 2Composition of casting liquid in all cases was identical to thatseti'orth in Example 1 and in cases the ratio of densiiiednitrocellulose particlesto casting liquid in the cast propellant grainswas approximate in diameter in all tests set forth above.

1 to 1 by weight.

The orifice employed in the vented vessel'firing tests'was 0.080 inch 7EXAMPLE A'sa'mple'of water-wet fibrous nitrocellulose of 12.6% nitrogencontent, 'densified in accordance with Example 1, was then beaten in alaboratory beater (D'owningtown Duplex Laboratory Beater, DowningtownIron Works, Downingtown, Pennsylvania) and that portion of the beatensample which passed through 35 mesh and was retained on 65 mesh wasselected for further testing. The bulk density of this sample was 30.4lb./cu. ft. (dry basis). Upon drying and casting in accordance withExample 1, a relatively soft grain having some porosity was obtained. A2 inch length of the resulting grain when fired in the customary ventedvessel firing test, employing a nozzle 0.100 inch diameter, gave thefollowing normal ballistic data:

Pressure in Lb./Sq. In. Burning Rate Burning Web Time (Inches),(Seconds) Burning Time P1 P: F; Max. (Seconds) EXAMPLE 6 Six hundredeighty grams (680), dry basis, of water-wet fibrous nitrocellulose of12.6% nitrogen content, after conventional preliminary stabilizationtreatment, but before jordaning, was slurried in 3275 grams of water ina vigorously agitated open vessel, and the slurry was heated to 75 C. bysparging steam into the slurry. One thousand two hundred grams (1200) ofmethyl ethyl ketone was then added to the heated agitated aqueousnitrocellulose slurry in about one minute, and heating was continuedwith agitation by' sparging steam into the slurry until the temperaturein the slurry reached 100 C. Substantially all of the solvent wasremoved by distillation, and the nitrocellulose was transformed, withoutbeing dissolved, from its initial fibrous state into smooth, hardened,densified, irregular particles having a bulk density (dry basis) of 25pounds per cubic foot. The resulting densified product, after drying,was suitable for use in casting grains of double-base propellant inaccordance with Example 1.

EXAMPLE 7 One hundred'fifty grams (150), dry basis, of water-wet fibrousnitrocellulose of 12.6% nitrogen content, after conventional preliminarystabilization treatment, but before jordaning, was slurried with 1350grams of water in a vigorously agitated 2-liter vessel fitted with acondenser. The mixture was heated to 100 C. with an electric heatingmantle around the vessel. Three hundred eight grams (308) of butylacetate was then added to the heated agitated aqueous slurry ofnitrocellulose in about 'two minutes, depressing the temperature toabout 91 C. Heating and agitation were continued until the tempera; tureof the slurry reached 100 C., requiring about 90 by distillation, andthe nitrocellulose was transformed, without being dissolved, from itsinitial fibrous state into smooth, hardened, densified, irregularparticles having a bulk density (dry basis) of 44 pounds per' 'cubiresulting densified product, after drying, was suitable for use .incasting grains of double-base propellant in accordance with Example 1.

c foot. 'The' 8 EXAMPLE 8 Forty parts (40), dry basis, of water-wetfibrous'nitrocellulose, 12% nitrogen type, and containing 17 partswater, was added to an agitated mixture containing 442 parts of Waterand 442 parts acetone at room temperature, and the slurry was agitatedfor four minutes. One thousand (1000) parts of water was then added andagitation was continued for one more minute. The liquid was drained andthe product was washed with water, then boiled in water for one hour,whereby substantially all solvent was removed, and the nitrocellulosewas transformed, without being dissolved, from its initial fibrous stateinto smooth, hardened, densified irregular particles having a bulkdensity (dry basis) of 34.1 pounds per cubic foot. The resultingdensified product, after drying, was suitable for use in casting grainsof double-base propellant in accordance with Example 1.

EXAMPLE 9 One hundred five parts dry basis, of water-wet fibrousnitrocellulose, 12% nitrogen type, and containing 45 parts water, wasadded to an'agitated mixture containing 893 parts of water and 96 partsof ethyl acetate at room temperature, and the resulting slurry wasagitated for five minutes. The liquid was drained off and 'the productquickly washed with 2000 parts of hot water. The product was then boiledin water for two hours to remove any residual solvent. Thenitrocellulose was transformed, without being dissolved, from itsinitial fibrous state into smooth, hardened, densified, irregularparticles having a .bulk density, dry basis, of 26 pounds per cubicfoot. The resulting densified product, after drying, was suitable foruse in casting grainsofdoublebase propellantin accordance with Example1.

While 12.6% nitrogen nitrocellulose is customarily employed in themanufacture of cast double-base propellant grains, the present inventionis by no means limited there- 'to, since all commercial types ofnitrocellulose can be densified 'by solvent granulation as set forthherein for the purpose of this invention. It is understood, of course,that both burning rate and temperature of burning are correlatable withthe nitrogen content of the nitrocellulose (other factors remainingconstant) with both burning rate and burning temperature increasing withincrease of nitr0- gen content in the "nitrocellulose. Thus, choice'ofnitrocellulose with respect to' nitrogen contentis one means forregulating and controlling the burning characteristics of castpropellant grains.

It has been found that this invention can be practiced withnitrocellulose-water slurries containing from about 3% to about 15% byweight of fibrous nitrocellulose and preferably from about 7% to about12% by weight of nitrocellulose. From an economic viewpoint it is, ofcourse, desirable to practice the invention with an aqueous slurrycontainingas much fibrous nitrocellulose as practicable. Although slurryconsistencies of less than 3% by weight of nitrocellulose can beemployed, it is not presently considered to be economical to do so. Theupper practical limit of nitrocellulose in the slurry is governed by theability to agitate the slurry effectively. The fibrous nitrocellulose tobe densified may be jordaned,or-othersomewhat smaller particlesareproduced when jord aned nitrocellulose is employed.

Solvent granulation and densification of nitrocellulose minutes.Substantially all of the solvent was'rernoyed in accordance with thisinvention depends upon the action of an active servant 'upo'rf'fibrousnitrocellulose suspended in water. The desired action is a softening'ofthe nitrocellulose fibers by the solvent toeliminatenthe fibrouscharacter thereof, witho'ut' actually dissolving-any measurable amountof "the nitrocellulose. Organic liquid solvents suitable for use in'practicing this invention. are those having an active solvent poweriornitrocellulose,

and preferably having an appreciable vaporpressure at or below theboiling point of water; Actually any active nitrocellulose solventcapable of subsequent removalby steam distillation can be employed fordensifying nitrocellulose for the purpose of this invention. Thesolvents may be soluble in water in all proportions or may have limitedsolubility in water. Suitable solvents include, by way of example,various ketones such as acetone, methyl ethyl ketone, methyl propylketone-methyl isobutyl ketone, and the like, various esters such asethyl acetate, propyl acetate, butyl acetate, and the like, and mixedether-alcohols such as mo'nomethyl ether of ethylene glycol, monoethylether of ethylene glycol, and the like. Butyl acetate and methylisobutyl ketone are presently preferred for densifying nitrocellulose inaccordance with this invention for use in casting double-basepro'pellant grains. The solvent treatment for densifying nitrocellulosecan be accomplished by addition of the solvent to an agitated slurry ofnitrocellulose in water, or by addition of nitrocellulose to an agitatedmixture of water and solvent, or by simultaneous addition of allingredients, with agitation, as in a continuous process.

In the solvent granulation and densification of nitrocellulose inaccordance with this invention, the ratio of active organic liquidnitrocellulose solvent to water is controlled in a range where enoughsolvent is partitioned to the nitrocellulose to soften and gel the tinyfibrous projections which are characteristic of nitrocellulose fiberswithout actually dissolving any measurable portion of thenitrocellulose. Too little solvent leaves the physical form of thenitrocellulose unaltered or not sufliciently altered, thus resulting ina product'having'low bulk density which causes fo'rmation of soft porouscastings. Too much solvent causes the product to'agglomcrate into largehard lumps which produce nonuniform castings. Still more solvent, ofcourse, causes undesirable dissolving of the nitrocellulose. The desired"degree of alteration of the physical structure of the nitrocellulose isto obtain a product which, after solvent removal, consists of smooth,hardened, densified, irregular shaped particles having a bulk density ofat least about 25 pounds per cubic foot (dry basis). It will be apparentthat solvent requirement to obtain thisobjective will vary somewhat withslurry consistency, initialphysical form of the nitrocellulose,temperature, degree of agitation, and solvent chosen for densificationpurposes. In general, however, it has been found that suitable productsfor the purposes of this invention are obtained by using between about15% and about 22% of either butyl acetate or methyl isobutylketone,based on combined weight of solvent and water in the aqueous slurry, andpreferably between about 16% and 19% by weight. The minimum suitablequantity of methyl ethyl ketone is on the order of approximately onepart by weight of methyl ethyl ketone for each 2.73 parts of water inthe aqueous nitrocellulose slurry. Similarly, the minimum suitablequantity of ethyl acetate is on the order of approximately one part byweight ethyl acetate for each 9.5 parts of water in the aqueousnitrocellulose slurry. When acetone is employed, the ratio isapproximately one part by weight of acetone to each part of water in theslurry. However, regardless of the solvent employed for densificationpurposes, it-is 'a simple expedient to carry out a preliminarydensification trial, using the hereinabove indicated quantities as aguide, and determine the degree of densification obtained, based on bulkdensity requirements. It will'be' apparent, of course, thatdensification, as measured by increased bulk density of the densifiedproduct, will improve with increasing .quantity of active nitrocellulosesolvent employed until that point is reached Where the softenedparticlesbegin to agglomerate into large lumps; This phenomenon governsthe upper useful limit of active solvent which can beemployed to alterthe physical structure of the nitrocellulose in accordance withthisinvention.

' into relatively narrow fractionswith respect to particle In practicingthis invention it has been found that softening of the nitrocellulosefibers by the activenitrocellulose solvent occurs almost instantaneously'upon bringing the nitrocellulose and solvent into contact with eachother in the aqueous slurry. Accordingly, it is important to maintainvigorous. agitation in the aqueous slurry from the moment that thenitrocellulose is contacted by the active solvent in the aqueous slurryuntil the softened nitrocellulose'particles have been hardened anddensified by removal of substantially all of the solvent. Agitationprevents agglomeration of the softened nitrocellulose particles andassures that the individual nitrocellulose particles retain theiridentity. Any desired additives, such as stabilizers, which are solublein the solvent used for densification and which are insoluble in watercan be introduced with the solvent and thus are distributed veryuniformly into the nitrocellulose product.

Densification of nitrocellulose by solvent granulation can be practicedat any ordinary temperatures. However, addition of solvent to theaqueous nitrocellulose slurry at or near the boiling point of thesolvent or its water azeotrope is desirable and is preferred because thepartition of solvent to the nitrocellulose is aided by reducedsolubility of the solvent in water. Moreover, solvent removal is greatlyaccelerated at elevated temperatures, and there is no tendency for thesoftened particles of nitrocellulose to putt up or popcorn duringsolvent removal, thus aiding materially in attaining a highbulk densityin the densified nitrocellulose product.

Hardening of the softened nitrocellulose is carried out by boiling offthe solvent or by diluting the aqueous slurry with water. Boiling ispreferred because in this manner particle hardening and solvent removalare accomplished simultaneously. When hardening is produced by dilution,subsequent solvent removal by draining and boiling is necessary. Solventremoval is carried out until substantially all solvent is removed fromthe nitrocellulose. This may require rom about 20 minutes to about 4hours, depending upon equipment used and the solvent which has beenemployed. Economical operation,

of course, dictates eflicient recovery of solvent for re-use in theprocess, and such recovery is readily achieved by distillation.

When the solvent strength 'is reduced by dilution with water or byboiling olf the solvent, the softened and gelled portions of thenitrocellulose harden to produce irregularly shapeddense granularparticles lackingthe fibrous projections which give ordinary fibrousnitrocellulose low bulk density and high compressibility.

The densified granular nitrocellulose produced in accordance with thisinvention should have a bulk density of at least about 25 pounds percubic foot, and preferably higher, since in general harder betterconsolidated cast double-base propellant grains are produced from higherdensity nitrocellulose particles. The ultimate upper limit for bulkdensity appears to be in the neighborhood of about 60 to 65 pounds percubic foot. Generally, the densified granular nitrocellulose productwill consist of particles having a relatively Wide diversity of sizes,as determined by screen analysis. It is presently believed that this isa desirable feature of the densified nitrocellulose particles of thisinvention, since the smaller particles help to fill in the spacesbetween larger particles,

thus leading to a-product of higher bulk density. :If desired, ofcourse, the densified nitrocellulose particles produced bythis inventioncan be classified by screening size, but the examples clearly indicatethat such classifica tion is not necessary forthe production of castdoublebase propellant grains having satisfactory ballistic char-.

acteristics. It is advisable, however, to screen out any particleslarger than 10 mesh, since such particles usual- 1y lead tononuniformity in the cast double-base propellant grains. Particleslarger than 10 mesh are conveniently 11 ground up and reworked throughthe solvent densification steps of this invention.

Following solvent removal, the water associated with the densifiedgranular nitrocellulose product is substantially all removed. This isusually accomplished by simple draining followed by conventional traydrying the moist product in warm air at about 140 F. The densifiednitrocellulose particles drain freely to about 20 to 40% (usually 20 to30%) moisture content, thus eliminating the need for dewateringcentrifuges.

The dry densified nitrocellulose particles are then charged into a moldof suitable configuration to conform with the shape of the propellantgrain desired and casting liquid is run into the mold followingconventional casting technique, employing either top or bottom filling,until all interstices between particles have been filled with castingliquid, whereupon the casting is cured, usually for several days atabout 140 F. to cause the mixture to set up and consolidate and weldtogether into a solid unitary homogeneous mass conforming to the shapeof the mold. This is brought about by the solvent action of the castingliquid on the nitrocellulose particles which absorb all of the castingliquid with resultant swelling and collodizing of the nitrocelluloseinto a homogeneous mass. After curing, the propellant grain is ready foruse in the rocket motor for which it is designed.

The proportion by weight of nitrocellulose and casting liquid in thecast propellant grain is governed largely by the bulk density of thegranulated nitrocellulose particles employed, the casting liquidemployed consisting of nitroglycerin and stabilizer, with or withoutvarious desensitizing plasticizers in accordance with conventional castdouble-base propellant practice.

The primary advantage of employing nitrocellulose densified inaccordance with this invention in the preparation of cast propellantgrains resides in the elimination of the tedious and expensive prior artprocess of converting fibrous nitrocellulose into uniformly shaped andsized grains of fully colloided material, requiring alcohol dehydrationof the nitrocellulose, colloiding, extruding, cutting, and solventremoval. Solvent densification practiced in accordance with thisinvention is much more economical than the conventional propellantpowder'manufacturi-ng processes which fireplaces. Other importantadvantages of solvent-densificatoin-of nitrocellulose reside in theelimination of the usual dewater'ing centrifuges from the nitrocellulosemanufacturing process, and in a marked simplification in thenitrocellulose stabilization process. -Further. advantages where thenitrocellulose is to be shipped reside in the reduced shipping costsresulting from the higher bulk densities and the much greater ease ofunloading the granular material from shipping containers as comparedwith tightly packed fibrous nitrocellulose.

This application is a continuation-in-part of my copending application,Serial No. 448,156, filed August 5, 1954, now abandoned.

What I claim and desire to protect by Letters Patent. is:

l. The process for preparing cast double-base propellant grains whichcomprises agitating fibrous nitrocellulose in an aqueous bath containingorganic liquid solvent which has active solvent power for saidnitrocellulose and which has an appreciable vapor pressure at 100 C.,said solvent being selected from the group consisting of solventssoluble in water in all proportions and solvents having limitedsolubility in water, said solvent being present in amount to soften anddestroy the fibrous structure of the nitrocellulose without dissolutionthereof, continuing agitation with removal of substantially all thesolvent until smooth, hardened, densified and irregular particles of thenitrocellulose having a bulk density on a dry basis of at 'least about25 pounds per cubic foot are obtained, removing substantially all Waterfrom the resulting densified nitrocellulose particles, charging theresulting dry densified nitrocellulose 'into a propellant grain castingmold, evacuating air therefrom, introducing a nitroglycerin castingliquid into the mold to completely fill all voids and just cover thenitrocellulose particles, and curing the mixture of nitrocellulose andcasting liquid in the mold until the particles of nitrocellulose absorball of the casting liquid and become swollen and fully colloidized andthe whole mass in the mold, due to the solvent action of the castingliquid on the nitrocellulose, consolidates and welds together into asolid, unitary, homogeneously colloided double-base propellant grainconforming to the shape of the casting mold.

2. The process for preparing cast double-base propellant grains whichcomprises agitating fibrous nitrocellulose in an aqueous bath at anelevated temperature up tothe boiling point of water, said bathcontaining organic liquid solvent which has active solvent power forsaid nitrocellulose and which has an appreciable vapor pressure at C.,said solvent being selected from the group consisting of solventssoluble in water in all proportions and solvents having limitedsolubility in Water, said solvent being present in amount to soften anddestroy the fibrous structure of the nitrocellulose without dissolutionthereof, continuing agitation with removal of substantially all thesolvent until smooth, hardened, densified and irregular particles of thenitrocellulose are obtained, drying the resulting densifiednitrocellulose particles, charging the resulting dry densifiednitrocellulose into a propellant grain casting mold, evacuating airtherefrom, introducing a nitroglycerin casting liquid into the mold tocompletely fill all voids and just cover the nitrocellulose particles,and curing the mixture of nitrocellulose and casting liquid in the molduntil the particles of nitrocellulose absorb all of the casting liquidand become swollen and fully colloidized and the whole mass in the mold,due to the solvent action of the casting liquid on the nitrocellulose,consolidates and welds together into a solid, unitary, homogeneouslycolloided double-base propellant grain conforming to the shape of'thecasting mold.

3. The process for preparing cast double-base propellant grains whichcomprises agitating fibrous nitrocellulose in an aqueous bath containingorganic liquid solvent which has active solvent power for saidnitrocellulose and which has an appreciable vapor pressure at 100 C.,said solvent being selected from the group consisting of solventssoluble in water in all proportions and solvents having limitedsolubility in water, said solvent being present in amount to soften anddestroy the fibrous structure of the nitrocellulose without dissolutionthereof, continuing agitation with removal of substantially all thesolvent by distillation until smooth, hardened, densified and irregularparticles of nitrocellulose having a bulk density on a dry basis of atleast about 25 pounds per cubic foot are obtained, drying the resultingdensified nitrocellulose particles having a bulk density on a dry basisof at least 25 pounds per cubic foot, charging the resulting drydensified nitrocellulose into a propellant grain casting mold,evacuating air therefrom, introducing a nitroglycerin casting liquidinto the mold to completely fill all voids and just cover thenitrocellulose particles, and curing the mixture of nitrocellulose andcasting liquid in the mold until the particles of nitrocellulose absorball of the casting liquid and become swollen and fully colloidized andthe whole mass in the mold, due to thesolvent action of the castingliquid on the nitrocellulose,

consolidates and welds together into a solid, unitary, homogeneouslycolloided double-base propellant grain conforming to the shape of thecasting mold.

4. The process for preparing cast double-base propellant grains whichcomprises agitating fibrous nitr0cellulose in an aqueous bath containingorganic liquid solvent which has active solvent power for saidnitrocellulose and which has an appreciable vapor pressure at 100 C.,said solvent being selected from-the group consisting of solventssoluble-inwater-in all-proportions and solvents having limitedsolubility in water, said solvent being present in amount to soften anddestroy the fibrous structure of the nitrocellulose without dissolutionthereof, continuing agitation with dilution of the aqueous bath byadding water thereto until smooth, hardened, densified and irregularparticles of nitrocellulose having a bulk density on a dry basis of atleast about 25 pounds per cubic foot are obtained and removingsubstantially all solvent by distillation, drying the resultingdensified nitrocellulose particles, charging the resulting dry densifiednitrocellulose into a propellant grain casting mold, evacuating airtherefrom, introducing a nitroglycerin casting liquid into the mold tocompletely fill all voids and just cover the nitrocellulose particles,and curing the mixture of nitrocellulose and casting liquid in the molduntil the particles of nitrocellulose absorb all of the casting liquidand become swollen and fully colloidized and the Whole mass in the mold,due to the solvent action of the casting liquid on the nitrocellulose,consolidates and welds together into a solid, unitary, homogeneouslycolloided double-base propellant grain conforming to the shape of thecasting mold.

5. The process for preparing cast double-base propellant grains whichcomprises agitating fibrous nitrocellulose in an aqueous bath at anelevated temperature up to the boiling point of water, said bathcontaining organic liquid solvent which has active solvent power forsaid nitrocellulose and which has an appreciable vapor pressure at 100C., said solvent being selected from the group consisting of solventssoluble in water in all proportions and solvents having limitedsolubility in Water, said solvent being present in amount to soften anddestroy the fibrous structure of the nitrocellulose without dissolutionthereof, continuing agitation with removal of substantially all thesolvent by distillation until smooth, hardened, densified and irregularparticles of nitrocellulose having a bulk density on a dry basis of atleast about 25 pounds per cubic foot are obtained, drying the resultingdensified nitrocellulose particles,'charging the resulting dry densifiednitrocellulose into a propellant grain casting mold, evacuating airtherefrom, introducing a nitroglycerin casting liquid into the mold tocompletely fill all voids and just cover the nitrocellulose particles,and curing the mixture of nitrocellulose and casting liquid in the molduntil the particles of nitrocellulose absorb all of the casting liquidand become swollen and fully colloidized and the whole mass in the mold,due to the solvent action of the casting liquid on the nitrocellulose,consolidates and welds together into a solid, unitary, homogeneouslycolloided' double-base propellant grain conforming to the shape of thecasting mold.

6. The process for preparing cast double-base propellant grains whichcomprises agitating fibrous nitrocellulose in an aqueous bath at anelevated tempera'mre up to the boiling point of water, said bathcontaining methyl isobutyl ketone in an amount sufficient to soften anddestroy'the fibrous structure of the nitrocellulose without dissolutionthereof, continuing agitation with removal of substantially all thesolvent by distillation until smooth, hardened, densified and irregularpanticles of nitrocellulose having a bulk density on a dry basis of atleast about 25 pounds per cubic foot are obtained, drying the resultingdensified nitrocellulose particles, charging the resulting 'drydensified nitrocellulose into a propellant grain casting mold,evacuating air therefrom, introducing a nitroglycerin casting liquidinto the mold to completely fill all voids and just cover thenitrocellulose particles, and curing the mixture of nitro-' celluloseand casting liquid in the mold until the particles of nitrocelluloseabsorb all of the casting liquid and become swollen and fullycolloidized and the .whole mass in the mold, due to the solvent actionof the casting liquid on the nitrocellulose, consolidates and weldstogether into a'solid, unitary, homogeneously colloided double-basepropellant grain conforming to the shape of the casting mold.

7. The process for preparing cast double-base propellant grains whichcomprises agitating fibrous nitrocellulose in an aqueous bath at anelevated temperature up to the boiling point of water, said bathcontaining butyl acetate in an amount sufiicient to soften'and destroythe fibrous structure of the nitrocellulose without dissolution thereof,continuing agitation with removal of substantially all the solvent bydistillation until smooth, hardened, densified and irregular particlesof nitrocellulose having a bulk density on a dry basis ofat least about25 pounds per cubic foot are obtained, drying the resulting densifiednitrocellulose particles, changing the resulting dry densifiednitrocellulose into a propellant grain casting mold, evacuating airtherefrom, introducing a nitroglycerin casting liquid into the mold tocompletely fill all voids and just cover the nitrocellulose particles,and curing the mixture of nitrocellulose and casting liquid in the molduntil the particles of nitrocellulose absorb all of the casting liquidand become swollen and fully colloidized and the whole mass in the mold,due to the solvent action of the casting liquid on the nitrocellulose,consolidates and welds together into a solid, unitary, homogeneouslycolloided double-base propellant grain conforming to the shape of thecasting mold.

References Cited in the file of this patent UNITED STATES PATENTS2,027,114 Olsen et a1. Jan. 7, 1936 2,035,471 Hale Mar. 31, 19362,292,469 Olsen Aug. 11, 1942 2,417,090 Silk et a1 Mar. 11, 1947

1. THE PROCESS FOR PREPARING CAST DOUBLE-BASE PROPELLANT GRAINS WHICHCOMPRISES AGITATING FIBROUS NITROCELLULOSE IN AN AQUEOUS BATH CONTAININGORGANIC LIQUID SOLVENT WHICH HAS ACTIVE SOLVENT POWER FOR SAIDNITROCELLULOSE AND WHICH HAS AN APPRECIABLE VAPOR PRESSURE AT 100*C.,SAID SOLVENT BEING SELECTED FROM THE GROUP CONSISTING OF SOLVENTSSOLUBLE IN WATER IN ALL PROPORTIONS AND SOLVENTS HAVING LIMITEDSOLUBILITY IN WATER, SAID SOLVENTS BEING PRESENT IN AMOUNT TO SOFTEN ANDDESTROY THE FIBROUS STRUCTURE OF THE NITROCELLULOSE WITHOUT DISSOLUTIONTHEREOF, CONTINUING AGITATION WITH REMOVAL OF SUBAND IRREGULAR PARTICLESOF THE NITROCELLULOSE HAVING A BULK DENSITY ON A DRY BASIS OF AT LEASTABOUT 25 POUNDS PER CUBIC FOOT ARE OBTAINED, REMOVING SUBSTANTIALLY ALLWATER FROM THE RESULTING DENSIFIED NITROCELLULOSE PARTICLES, CHARGINGTHE RESULTING DRY DENSIFIED NITROCELLULOSE INTO A PROPELLANT GRAINCASTING MOLD, EVACUATING AIR THEREFROM, INTRODUCING A NITROGLYCERINCASTING LIQUID INTO THE MOLD TO COMPLETELY FILL ALL VOIDS AND JUST COVERTHE NITROCELLULOSE PARTICLES, AND CURING THE MIXTURE OF NITROCELLULOSEAND CASTING LIQUID IN THE MOLD UNTIL THE PARTICLES OF NITROCELLULOSEABSORB ALL OF THE CASTING LIQUID AND BECOME SWOLLEN AND FULLYCOLLOIDIZED AND THE WHOLE MASS IN THE MOLD, DUE TO THE SOLVENT ACTION OFTHE CASTING LIQUID ON THE NITROCELLULOSE, CONSOLIDATES AND WELDSTOGETHER INTO A SOLID, UNITARY, HOMOGENEOIUSLY COLLOIDED DOUBLE-BASEPROPELLANT GRAIN CONFORMING TO THE SHAPE OF THE CASTING MOLD.